1. Field of the Invention
The present invention relates to an image recording apparatus which records an image on a recording medium and a method by which the image is recorded on the recording medium.
2. Description of the Related Art
An image recording apparatuse such as an ink jet printer attaches ink on a recording medium such as a paper to record an image thereon. The image recording apparatus includes a recording head which ejects ink to the recording medium, a carriage which holds the recording head, a recording medium transfer mechanism which transfers the recording medium, and carriage driving mechanism which moves the recording head along an axial line (a main scanning direction MA) crossing an axial line (a sub-scanning direction SA) along a transfer direction of the recording medium by the recording medium transfer mechanism at right angles.
Such conventional image recording apparatus as described above drives the carriage in the main scanning direction MA, and makes the recording head moved together with the carriage eject ink toward the recording medium. With this operation, the conventional image recording apparatus applies the ink drops from the recording head to the recording medium at a substantially equal pitch in the main scanning direction MA and records a part of a desired image for a width of the recording head (that is, a size of the recording head in the sub-scanning direction SA) on the recording medium. The image recording apparatus repeats the above-described operation to the recording medium intermittently transferred in the sub-scanning direction SA and records a whole of the desired image on the recording medium.
Generally in the image recording apparatus, the temperature of the recording head chages while the recording head records the image, due to heat generation of members (piezoelectric element, electrode, driving IC, etc.) constituting the recording head. For example, a rise in the temperature of the recording head during the image recording lowers the viscosity of ink in the recording head gradually. Lowering of the viscosity of ink in the recording head increases an amount of one ink drop ejected from the recording head at one time. Therefore, a diameter of an ink dot attached on the recording medium increases. The recording head ejects ink while it moves in the main scanning direction, and records the image on the recording medium. Since the temperature of the recording head changes while the recording head moves as described above, in the image recorded on the recording medium, an optical density of a part of the image at a start of image recording and that at an end of the image recording are different from each other in one scanning operation, as shown in FIG. 12. Especially, as shown in FIG. 12, when the image recording is performed bidirectionally along the main scanning axial line, a density difference between upper and lower scanning bands becomes remarkable, and a quality of the image recorded on the recording medium is degraded remarkably.
For example, even if, while the image recording apparatus is scaned a plurality of times and records the image on the recording medium, an equal voltage is applied to the recording head to record the image by each scanning, the optical density (OD) of the image recorded by one scanning operation changes, as shown in FIG. 13. FIG. 13 is a graph showing the changes in the optical density of the image in a recording order (that is, a scanning order) in a case where the image is recorded on the recording medium by a plurality of scanning operations of the recording head at a half tone of 60%. As shown in FIG. 13, there is a difference in the optical density between the part of the image at the start of the image recording and that at the end of the image recording in one scanning operation. Concretely, the optical density (OD) of the image increases gradually from the start of the image recording to the end of the image recording. As described above, in the conventional image recording apparatus, the optical density of the image recorded on the recording medium changes in the moving direction of the carriage, and the quality of the recorded image is degraded.
And, since the temperature of the recording head changes rapidly, it is difficult to detect the change in the temperature correctly in real time by a temperature detection element such as a thermistor disposed in the recording head.
In recent years, an image recording apparatus which intends to stabilize the temperature of the recording head has been proposed. Such conventional image recording apparatus as described above is described, for example in Japanese Patent Application KOKAI Publication No. 5-64890, especially in FIG. 5.
In the image recording apparatus of the Publication, a heater is connected to the recording head. And, the change in the temperature of the recording head during the image recording is predicted, and the temperature of the recording head during the image recording is stabilized by the heater.
In another conventional image recording apparatus, while the recording head scans once, a driving voltage applied to the recording head is uniformly lowered to prevent the optical density in the image recorded from the start of the image recording to the end of the image recording in one scanning operation of the recording head from becoming uneven.
In further conventional image recording apparatus, instead of uniformly lowering the driving voltage, image data to be used to record an image is obtained before recording the image. And, while the recording head scans once, the driving voltage applied to the recording head is lowered in accordance with a gradation value of each position of the image data to prevent the optical density in the image recorded from the start of the image recording to the end of the image recording in one scanning operation of the recording head from becoming uneven.